1. Field of the Invention
This invention relates to a paper transfer mechanism for use in an image forming apparatus for forming a toner image, corresponding to image data, through an electrostatic copying process, and forming the toner image on a sheet of paper.
2. Description of the Related Art
In general, a laser beam printer apparatus, a copying machine or another image forming apparatus has an image forming section for forming a toner image, corresponding to image data, on a sheet of paper, a fixing unit for fixing the toner image formed on the paper sheet, a discharge unit (paper transfer mechanism) for discharging the paper sheet on which the toner image is fixed, and a paper-discharging tray for stacking therein discharged paper sheets.
The discharge unit includes a driving roller unit having a tread surface and a pressing roller unit having a tread surface arranged in contact with that of the driving roller unit. A sheet of paper is transferred to the discharge tray through the roller elements of the roller units.
A sheet, on which a toner image has been fixed by the fixing unit, is usually waved due to heat transmitted from the fixing unit.
Such waved sheets are received inevitably confusedly in the discharge tray, and hence it is possible that sheets are hard to take out of the tray, when they are continuously discharged.
If sheets are greatly waved, a first-discharged sheet may collide with the next-discharged one. As a result, the former may be pushed by the latter and scattered around the tray, or may be inserted between already stacked sheets.
To avoid the above, it has been proposed that a sheet is waved in a direction perpendicular to the direction of transfer. This technique is called "corrugation".
The corrugation is performed by a corrugation roller unit which is formed coaxial with one of the driving roller unit and the pressing roller unit, and has a diameter larger than the roller of the one of the units.
The corrugation roller unit is assembled integral, for example, with the driving roller unit. The number of corrugation rollers employed in the corrugation roller unit and the distance between each adjacent pair of the corrugation rollers are determined on the basis of the size of a paper sheet which can be transferred by the discharge unit.
The corrugation roller unit has a configuration, a structure and a function as disclosed, for example, in Jpn. UM Appln. KOKAI Publication No. 62-32061. FIGS. 6a and 6b of the publication show corrugation rollers 57a, 53a, 57b, 53b, 57c, 53c and 57d which have a diameter larger than transfer rollers 56a and 52a, and 56b and 52b (the outer peripheries of the rollers 56a and 52a (or the rollers 56b and 52) contact each other).
In the case of using the corrugation roller unit, however, if a paper sheet is thick, it is possible that each corrugation roller has its track put on the sheet in the direction of transfer.
To avoid this, an improved corrugation roller unit has been proposed, in which the boss of each corrugation roller has a diameter larger than the shaft of the driving roller, and the corrugation roller is attached to the driving roller via a compressed coil spring. The configuration, structure and function of this corrugation roller unit is described in detail, for example, in Jpn. Pat. Appln. KOKAI publication No. 3-101979.
In this corrugation roller unit, if a paper sheet to be discharged is thinner than a predetermined thickness, the outer periphery of the corrugation roller projects from that of the driving roller by means of the compressed coil spring, thereby corrugating the sheet.
In this corrugation roller unit, if the sheet is thicker than the predetermined thickness, the thickness (i.e., hardness) of the sheet pushes back the compressed coil spring and restrains the projection of the outer periphery of the corrugation roller. This enables the sheet to be discharged without the track of the corrugation roller.
This corrugation roller unit, however, has a complicated structure. Therefore, a large number of component parts and a large number of assembling steps are necessary. Inevitably, a great manufacturing cost is required.